1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for determining an offset for marine seismic acquisition while surveying a subsurface.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration for determining the properties of earth's subsurface, which are especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional marine system for generating the seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The streamers may be disposed to have other spatial arrangements than horizontally. The vessel 10 also tows a seismic source 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downward toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until is detected by the receiver 11 on streamer 12. Based on the data collected by the receiver 11, an image of the subsurface is generated.
In order to obtain an accurate image of the subsurface being surveyed, it is desirable to have accurate information about a distance between the source 16 and each of the receivers 11A and 11B (see FIG. 2). In this way, the recorded data can be accurately matched to the various points of the subsurface. Traditionally, the distance between the source and a receiver (offset herein) is determined as follows. As shown in FIG. 2, the back end 12A of the streamer is attached to a buoy 40 that floats at water surface 14. The buoy 40 may have a GPS device 42 that determines the location of the buoy. The source 16 may include a GPS device 44. The same is true for the vessel 10, i.e., it also includes a GPS device 46. By measuring the position of the source 16 with the GPS 44 and the position of the end of the streamer with the GPS device 42, and by knowing the length of the streamer and the placement of the receivers 11A and 11B along the streamer 12, the position of each receiver relative to the source 16 may be calculated. Moreover, an acoustic positioning network may be used to determine the underwater relative positions of the receivers, thus helping to measure the positions more accurately. It is noted that FIG. 2 shows the subsurface 50 that is being surveyed below the body of water 52.
The four-dimensional (4D) geophysical imaging, which is becoming more desired today, requires a high accuracy of the offset, e.g., below 1 meter. In this regard, it is noted that the length of a streamer is about 10 km and, thus, an offset may have a value between 100 m to 10,000 m. The 4D geophysical imaging involves 3D seismic surveys repeated over a same subsurface at different moments in time to determine changes in the geophysical structures of the subsurface. Thus, as the 3D survey is repeated in time, sometimes after a couple of months or years, it is desirable that the offsets are as close as possible to the offsets in the previous survey over the subsurface.
However, the currently used GPS and acoustic devices have their limitations and, for various surveys (e.g., the 4D surveys), the accuracy of these systems might not be good enough, especially for the baseline survey and early monitoring surveys, which were acquired with less accurate positioning system. Alternatively, there is a need to verify the results provided by the GPS and/or acoustic devices. This verification would be especially helpful during the acquisition.
Accordingly, it would be desirable to provide alternative systems and methods that do not rely on GPS or acoustic devices for determining/verifying the offsets during a marine seismic survey.